1. Field of the Invention
The present invention relates to a ceria electrolyte for low-temperature sintering of a solid oxide fuel cell (SOFC) and, more particularly, to a ceria (CeO2) electrolyte and a solid oxide fuel cell using the same, in which the ceria electrolyte is configured such that either gadolinium (Gd) or samarium (Sm) is co-doped with ytterbium (Yb) and bismuth (Bi), thus exhibiting low-temperature sintering properties.
2. Description of the Related Art
Recently, in order to reduce the operating temperature of a solid oxide fuel cell (SOFC) to ensure the high-temperature stability thereof and to prevent the power of the solid oxide fuel cell from decreasing due to the reduction in the operating temperature, high-performance materials are being actively applied. Currently widely useful as an electrolyte material for a unit cell for a solid oxide fuel cell is a zirconia-based electrolyte such as 8YSZ (8 mol % Y2O3 stabilized ZrO2).
Initially, with regard to unit cells using a zirconia-based electrolyte such as 8YSZ, a cathode material contains a composite comprising LSM (Sr-doped LaMnO3), as a pure electron conductor, and an electrolyte material. Recently, however, to increase the power density (W/cm2) of the unit cell, the use of a mixed ionic and electronic conductor (MIEC) such as LSCF (Sr- & Co-doped LaFeO3), having superior oxygen ionization catalytic properties and high electrical conductivity even at low temperatures, is drastically increasing as the cathode material.
However, most MIEC cathode materials such as LSCF are problematic because they may react with the zirconia electrolyte at the interface therebetween in the thermal treatment temperature range of the cathode, undesirably forming non-conductive reaction products. Hence, in order to prevent the MIEC cathode such as LSCF and the zirconia electrolyte from reacting at high temperatures, a buffer layer (BL) is additionally provided between the cathode and the electrolyte.
In particular, a ceria-based electrolyte (Gd- or Sm-doped CeO2) comprising pure CeO2 and 5 to 10 mol % of Gd2O3 or Sm2O3 has high oxygen ionic conductivity and does not react with MIEC cathodes, and is thereby widely utilized as a material for a buffer layer, which is interposed between the zirconia (ZrO2)-based electrolyte membrane of the solid oxide fuel cell and the MIEC cathode layer so as to prevent the production of a reaction product between the electrolyte and the cathode.
Useful as the buffer layer, a ceria-based electrolyte is characterized by forming an isomorphous solid solution with a zirconia electrolyte at a high temperature of 1300° C. or higher, in which the solid solution, formed at a high temperature through mutual diffusion, has very low ionic conductivity, consequently deteriorating the power of the unit cell.
The fabrication of a unit cell using a ceria-based buffer layer includes two types of methods, one method including forming an anode support, a zirconia-based electrolyte and a ceria electrolyte into a laminated molded body that is then co-sintered, and the other method including forming an anode support and a zirconia electrolyte into a laminated molded body that is then sintered, coating the surface of the electrolyte membrane with a ceria buffer layer, and then thermally treating it.
Thus, when the unit cell is manufactured by co-sintering the ceria-based electrolyte and the zirconia electrolyte, co-sintering at a temperature of 1300° C. or lower is required, but the zirconia electrolyte has to be subjected to a sintering temperature of 1350° C. or higher in order to attain the dense microstructure of 95% or more typically required of electrolytes.
Also, when the ceria-based buffer layer is formed by preparing a dense electrolyte through sintering at 1350° C. or higher and coating and thermally treating the surface of the electrolyte, the already-sintered electrolyte membrane does not shrink, undesirably making it difficult to form a dense ceria-based electrolyte and increasing the processing time and cost due to the additional coating and thermal treatment.
Supposing that the thermal treatment temperature of the ceria-based electrolyte used as the buffer layer is lowered to the thermal treatment temperature of the cathode, the ceria-based electrolyte and the cathode may be thermally treated simultaneously, thus reducing the processing time and ensuring a dense microstructure even at a low temperature, thereby improving the power characteristics of the unit cell.
Therefore, techniques for decreasing the thermal treatment temperature of the ceria-based electrolyte to 1150° C., corresponding to the thermal treatment temperature of the cathode, are required.